The invention relates to an analysis system for the identification of the contents of objects, particularly explosives and/or chemical warfare agents, with a neutron source that generates neutrons which act on the object and cause the emission of characteristic xcex3 quanta from atomic nuclei of the contents of the object, with a detector for detection of the emitted xcex3 quanta and an electronic measuring and signal processing system for allocating the detected signals to certain atomic nuclei and for detecting certain chemical compounds which contain these atomic nuclei.
Such an analysis system is known from the publication xe2x80x9cNeutron Activation Analysisxe2x80x9d issued by the defense office of the German Federal Army for ABC Protection (WWD No. 150, 1994), to the entire content of which reference is made.
Particularly in connection with the worldwide problem of the disposal of chemical warfare agents and ammunition, but also the identification of explosives, methods of non-destructive analysis and identification are of very great interest.
One possible method is neutron activation analysis which can differentiate between various explosives and warfare agents, for instance, using the detected element concentrations.
In this method, a neutron source releases neutrons which penetrate the object being examined and interact with the atomic nuclei inside. Neutron activation analysis uses the xcex3 radiation emitted due to the nuclear reaction to determine the composition of the object being examined or its contents. Due to the nuclear reaction, xcex3 quanta of discrete energy or energies are emitted which are specific to the element atoms participating in the nuclear reaction. Due to an energy-dispersive detection of the xcex3 quanta and corresponding evaluation of the energy spectrum, element analysis of the contents of a container, for instance, can be performed. Analysis is independent of the aggregate condition of the container contents; chemical composition (decomposition processes due to aging) or spatial separation of substances also have little or no effect on the result of measurement.
An overview of the principles of the present invention can be found in the book entitled xe2x80x9cKern- und Elementarteilchenphysikxe2x80x9d (Nuclear and high energy physics) by Musiol et al. (VCH, 1988): Chapter 12.3 xe2x80x9cMaterial and Process Analysis With Nuclear Radiationxe2x80x9d, particularly 12.3.5. xe2x80x9cActivation and Excitationxe2x80x9d.
In the publication cited at the outset, it is initially explained that there are in principle two classes of neutron activation analysis (NAA): delayed classic NAA and prompt xcex3 activation analysis (PGAA). With the first method the object is irradiated with neutrons and therefore nuclei are activated, and in a subsequent, spatially separated second step the xcex3 radiation emitted by the activated nuclei is measured. Between these two steps there is a waiting or transport period of usually several minutes to some hours, but at least a few seconds. In one embodiment, investigated sample material is transported from the neutron source to the detector via a pneumatic tube conveyor (see FIG. 6 in the cited publication). Due to the spatial separation, one avoids direct influence of the neutron source on the detector. However, the sample becomes radioactive due to this treatment. In the second method, the xcex3 quanta emitted directly upon neutron scatter/capture are detected in a single-stage process. This has the advantage that almost all the elements can in principle be detected because one is not dependent on unstable isotopes. However, now the source and detector must be close together, which leads to problems due to scattered radiation.
Neutron sources can certainly be any of the following: nuclear reactors; neutron generators where deuterons, for example, are shot at a target made of tritium (stationary); and isotope neutron sources (mobile). FIGS. 8a and 8b in this document show arrangements using isotope sources which are based on thermal neutron capture (8a) and inelastic neutron scattering (8b).
The described systems are either stationary and, at least as far as the source is concerned, bound to a corresponding set-up or can be mobile, but then they include a radioactive isotope source. Regular neutron generators using tritium, also use a radioactive material which is potentially very dangerous to human beings.
Particularly for mobile operation to examine objects, for example in ammunition depots, where the neutron source has to be moved by various means of transportation, any radioactive component constitutes a hazard which leads to increased expenditure on safety or renders certain applications absolutely impractical.
Therefore there is a need for a mobile analysis system of the above type where such a hazard is reduced or eliminated.
The task is solved by having the analysis system consist of a mobile frame to which the neutron source and the detector as well as the object holder are attached, the neutron source is a neutron generator which contains a deuterium target and, by periodic pulsed bombardment of the target, generates neutron pulses and can be controlled so that the neutron pulses are emitted in the energy range of 2.5 MeV from 1 xcexcs to 1 ms duration, preferably between 20 xcexcs and 50 xcexcs, and are repeated at a cycle time of between 5 xcexcs and 100 ms, the detector is controllable in such a way that it detects xcex3 quanta promptly emitted from the object due to inelastic neutron scattering and neutron capture, in a range between 30 keV and 11 MeV within at least two consecutive temporal measurement windows in cycles, whereby the first measurement window at least partially has a temporal overlap with the neutron pulse and the following, second measurement window not, which means that in the first measurement window essentially xcex3 quanta are detected due to inelastic neutron scattering and in the second measurement window they are detected due to neutron capture.
Due to the attachment of the source and detector to a common frame the system can be made mobile and compact. Use of a pulsed neutron generator with a deuterium target ensures that no radioactive materials are present and the system does not constitute any hazard when the generator is switched off. Utilization of prompt xcex3 activation analysis (PGAA) makes it feasible for practically no radioactive isotopes to be generated in the object being examined and so it remains safe for subsequent handling. Signal reception in two measurement windows, which largely correspond to the two processes taking place, increases the selectivity and accuracy of identification and reduces total measuring time.
Attention must be drawn to the fact that, despite the second-window detection, which is delayed in the micro- to millisecond range, analysis is still PGAA. The delay results from several upline inelastic scattering processes which reduce the neutron energy so that neutron capture is made possible. In the energy spectrum of the first detection window, xcex3 lines which correspond to inelastic neutron scattering tend to dominate, while in the second detection window it is the xcex3 emission lines which dominate after neutron capture. Therefore, the xcex3 spectra can be evaluated separately according to the types of nuclear reaction. Consequently, superpositions of energy lines made up of different types of nuclear reaction are largely avoided and particularly the xcex3 spectra of the second detection window have a low xcex3 background. Line allocation, peak area calculation and therefore determination of the involved types of nucleus can be advantageous. Typically, for examination of an object there is an accumulation over very many measuring cycles (in the ms range), so total measuring times of minutes can result. Obviously, the measurement can be automatic or semiautomatic. In particular, abort criteria can be written into the software as soon as adequate reliability of the identification result is achieved.
It is advantageous if there is a shield against direct xcex3 radiation between the neutron generator and the detector. This eliminates scattered radiation which would otherwise constitute a problem due to the close proximity. It has become evident that for the intended purposes a shield made of tungsten is most suitable. Apart from the direct admission of xcex3 radiation to the detector, neutrons which can pass directly from the source into the detector can also cause interference. It is therefore also preferable to provide a shield against such neutrons, for instance in the form of small cadmium plates. Since xcex3 quanta can certainly be generated again in cadmium, it is particularly preferable if the cadmium plates are for the most part completely surrounded by tungsten, which therefore also protects the detector against the xcex3 radiation resulting in the cadmium.
Preferably, the detector is a solid state detector (HPGe) with cooling, particularly by coupling to a bath of cryogenic liquid such as nitrogen or by means of a refrigerator. A Peltier cooling system can in principle also be used.
Due to the development of reliable compact coolers, the utilization of electrically driven refrigerators would seem particularly advantageous. The cooled solid state detectors have a much better resolving power than scintillation detectors.
The detector is connected to an electronic amplifier unit with, for example, preamplifier, main amplifier and ADC, a four-channel analyzer, and a computer (PC) with evaluation software for peak analysis of the recorded xcex3 spectra. The computer can also handle the controlling of the entire measuring procedure, i.e. it can essentially control pulse length, cycle times, total measuring time, amplifier settings, etc.
The computer memory device contains the peak positions and other parameters of set elements for the relevant nuclear processes. Preferably, analysis of the measured xcex3 spectra uses at least two of the following elements: hydrogen, nitrogen, aluminum, fluorine, phosphor, sulfur, chlorine, or arsenic, which are characteristic of many chemical warfare agents or explosives.
It is particularly advantageous that the frame of the mobile analysis system includes devices for adjusting the neutron generator and the detector. In this way it is always possible in a compact unit to set the best geometry for the intended measurement. It is also preferable if the shielding and/or sampling are adjustable.
The frame, electronics and detector cooling should preferably be accommodated, at least partially, in a common housing. The computer may also be integrated in the housing, but connection of an external laptop computer is also a useful alternative. All this leads to a compact, robust and mobile unit which, without any major difficulties, can be taken to various sites in the field by any means of transportation.
Apart from the shield between the neutron source and the detector, the analyzer can include a shield to protect operating personnel against neutrons and xcex3 radiation. This makes it possible for the personnel to be close to the unit during the current measurement.
The examined objects should preferably be metal-cased containers (warfare agents), grenades, bombs, or the like. The entering neutrons and the emitted xcex3 quanta penetrate the metal case.
It is evident that the features described above and those listed below can not only be used in the mentioned combination but also in any other combination or on their own, without abandoning the scope of the invention.
The invention will be explained in more detail using the embodiments and figures. The figures are as follows: